Method of Producing Microparticles

ABSTRACT

A method of producing microparticles having a median diameter up to 100 μm and the microparticles so produced are described. The method includes the steps of providing a solvent having a bioactive dispersed or dissolved therein and a vehicle dissolved therein, carrying out an emulsification in a non-solvent phase to produce an emulsion containing the bioactive and the vehicle in a solvent phase, and evaporating the solvent to leave the microparticles, wherein a mixture of at least two surfactants is employed to stabilize the emulsion and wherein the mixture has a hydrophilic-lipophilic balance (HLB) of up to 8.

The present invention relates to a method of producing microparticles,in particular microparticles of a drug encapsulated by a polymer whichallows for delayed and/or extended drug release in the gastrointestinaltract.

The concept of using pH-sensitive polymers to target drugs tosite-specific regions of the gastrointestinal (GI) tract is not a newone. Gastric irritant or labile drugs are routinely administered asenteric coated tablet or pellet systems, and by choosing a polymer witha suitably high dissolution threshold pH, it has been attempted totarget the terminal ileum/colon region for the treatment of inflammatorybowel diseases specific to this area.

However, these methods are not without their limitations. The large sizeof these systems normally results in delayed gastric emptying,especially when administered after a meal, which will result in delayedand unpredictable onset of drug action. The GI transit time of largemonolithic systems are also subject to more variation than those ofmultiparticulate systems and this can lead to variation inbioavailability.

Due to their small size, microparticles would be expected to suspend inthe gastric contents and therefore empty rapidly through the pylorus inboth the fed and fasted state. Transit through the small intestineshould be more reproducible, and transit through the colon should beslower, reducing the chances of a colon-targeted dosage form beingvoided intact. The large surface area of a microparticulate systemshould also allow a faster drug release once the pH threshold isreached. Drug dissolution is therefore expected to be more rapid, aparticular advantage for drug targeting to the colonic region given thelimited fluid volume in this area. Regarding the potential for the colonas a site for protein and peptide delivery, microencapsulation may be apreferable method of getting such drugs into a delivery system, placingmuch less mechanical stress on these labile molecules than thepreparation of pellets or tablets would. A wide range of modifiedrelease polymers are used as coatings for tablets, pellets and capsulesby the pharmaceutical industry for extended and delayed releaseformulations. Examples of extended release polymers are the cellulosederivatives ethylcellulose and cellulose acetate, theammoniomethacrylate copolymers (e.g. Eudragit RS and RL), and polyvinylacetate. For delayed release, polymers are generally soluble above athreshold pH, which corresponds to the pH of a certain region within thegastrointestinal tract (e.g. Eudragit L100-55 (pH 5.5) and L100 (6.0)for intestinal targeting and Eudragit S100 (7.0) and P4135 (7.0-7.4) forcolonic targeting). In particular, previous attempts at formulatingmicroparticles of Eudragit L100 and Eudragit S100 have beenunsuccessful, resulting in particles of poor morphology and control ofdrug release, and have involved complicated production methods involvinghomogenisation, careful control of temperature (for production of a goodemulsion and/or solvent removal) or rate of addition of surfactant (Gotoet al (1986), Morishita et al (1991), Squillante et al (2003).

Given the theoretical advantages of microparticulate systems overconventional dosage forms, the present applicant decided to try toovercome the problems that have led to the production of microparticlesof poor morphology and control of drug release. It was decided tooptimise the emulsification/solvent evaporation method for theproduction of Eudragit L/S100 microparticles, a commonly used method ofmicroencapsulation and to apply this optimised method to other modifiedrelease polymers.

The emulsification/solvent evaporation method is a conceptually simple,three step process.

In step one, polymer is dissolved in a suitable solvent (into which thedrug is dispersed, or preferentially dissolved). This solvent is alsoknown as the “internal phase”. The solution of drug and polymer is thenemulsified into a non-solvent (or “external”) phase usually containing asurfactant to improve emulsion stability.

In step two, solvent is allowed to evaporate, usually under agitation.

In step three (once step two is complete), particles are solidified, andcan be separated by filtration and cleaned up.

The formation of a stable emulsion in the early stages is important ifdiscrete microparticles are to be isolated. It has also been found thatthe choice of solvent influences microparticle morphology depending onthe rate at which it migrates from the polymer solution into thenon-solvent phase and is removed by evaporation. The solubility of thepolymer in the chosen solvent and boiling point are factors that affecthow quickly the particles solidify. During this process the forming“particles” will evolve from being liquid emulsion droplets, tosemi-solid “sticky” particles, to solidified, discreet particles. Thelength of time the particles exist in the semi-solid form is expected toinfluence coalescence of the forming particles and the overallmorphology of the end product.

Previous attempts at microencapsulation of Eudragit L100 and S100 haveresulted in particles of poor morphology (see Goto et al. and Morishitaet al.).

Acta Technologiae et Legis Medicamenti, 2003, 14(1), 53-66 (Mateovic)discloses the preparation of microspheres of Eudragrit® RS by means ofthe solvent evaporation method using the following surfactants:magnesium stearate, Span 20, and a combination of magnesium stearate andSpan 20. The resulting microspheres were sieved and drug content anddissolution were determined for the 315-400 μm fraction. However, theparticles produced by this method do not function as intended. Evenregarding the bioadhesive polymer content, the water insoluble EudragitRS is clearly intended to extend the release of drug from the particles,but in fact 100% of the drug is released within a one hour period. Theformulation behaves therefore as an immediate release formulation (thedefinition of which is 70% release within 45 minutes).

In accordance with the first aspect of the present invention, there isprovided a method of producing microparticles comprising a bioactive anda vehicle, which method comprises providing a solvent having a bioactivedispersed or dissolved therein and a vehicle dissolved therein, carryingout an emulsification in a non-solvent phase to produce an emulsioncomprising the bioactive and the vehicle in a solvent phase, andevaporating the solvent to leave said microparticles, wherein a mixtureof at least two surfactants is employed to stabilise said emulsion andthe HLB (hydrophilic-lipophilic balance) of the mixture is up to 10.

Up to now, there been a problem controlling drug release when using pHdependent release polymers. In order to solve this problem, it has beenassumed that the microparticles have to be relatively large (perhapshaving median diameters in the order of millimetres) in order to deliverloaded drug effectively in vivo. This is because smaller particles witha relatively large surface area are thought to release drug far tooquickly in acid conditions. The present inventors have made thesurprising discovery that improved drug dissolution rates can beobtained by forming microparticles of much smaller dimensions. Inparticular, it has been found that microparticles having a mediandiameter of up to 100 μm, preferably from 20 to 60 μm and mostpreferably from 30 to 50 μm can be produced by means of the inventivemethod.

It will be appreciated that emulsification methods such as the presentmethod result in a range of microparticles of varying sizes. Inparticular, the diameter of the particles is likely to adopt a so-called“normal distribution”, with very few particles having extreme diametersand the majority having average diameters. Thus some prior art methodsmay well result in some microparticles having diameters of 100 μm orless. However, it is believed that there are not any prior art methodswhich result in a distribution of microparticles in which the medianparticle diameter is 100 μm or less.

The HLB of the mixture of surfactants is preferably up to 8, morepreferably from 2 to 7 (alternatively from 2 to 5) and most preferablyfrom 3 to 5 (alternatively from 3 to 4).

By “surfactant” is meant a molecule having a hydrophobic portion whichis a hydrocarbon chain and a hydrophilic portion such as pendent ionicor polar groups. Thus when carrying out an oil-in-water or water-in-oilemulsification process, the surfactant molecule can become orientated sothat the hydrocarbon (or “fatty”) portion interacts with the oil phaseand the polar/ionic (“non-fatty”) portion interacts with the waterphase, thereby stabilising the emulsion. Such a molecule is referred toas “amphiphilic” (because it interacts with both polar and non-polarmolecules) and by “surfactant” herein is meant “amphiphilic surfactant”.It does not encompass non-amphiphilic surfactants such as anti-foams.

In simple terms, HLB is the mole percentage of the hydrophilic portionof the surfactant molecule divided by 5. Therefore a completelyhydrophilic molecule will have an HLB value of 20, and a completelyhydrophobic molecule will have an HLB value of zero. Most surfactantsare amphiphilic and will have HLB values between 0 and 20, enabling themto orientate at the interface between the two phases of an emulsion,thus stabilising said emulsion as explained above.

The mixture of surfactants is preferably an equimolar mixture of onlytwo surfactants, and may comprise sorbitan monoleate and sorbitandioleate. Other surfactant combinations include Tween 80 and Span 85;Span 80 and Span 85; Span 85 and Span 20; Span 80 and Span 20; or acombination of any two thereof.

In a particularly preferred embodiment, sorbitan sesquioleate is used asa surfactant to stabilise said emulsion. Sorbitan sesquioleate isavailable from Uniquema under the trade name Arlacel 83 and is anequimolar mixture of sorbitan monoleate and sorbitan dioleate.

Without wishing to be constrained by theory, one possible explanation isthat it is the combination of two or more surfactants (in the case ofArlacel 83, an equimolar combination of sorbitan monoleate and sorbitandioleate) which functions on a molecular level to stabilise theemulsion. It is thought however that the composite surfactant shouldstill have an HLB in the appropriate range.

In an alternative embodiment, said mixture of at least two surfactantsdoes not include Span 20 and Span 80 in combination.

The vehicle may be a polymer which enables (preferably) pH-dependentand/or pH-independent release of the bioactive in the gastrointestinaltract. Examples of preferred classes of polymer are acrylic-basedpolymers (such as methacrylate), cellulose-based polymers orpolyvinyl-based polymers. By “based” is meant that a portion (preferablya substantial part) of the polymer chain comprises said group.

Particularly preferred polymers comprise Eudragit® L100, Eudragit®L100-55, Eudragit® S100, Eudragit® P4135, Eudragit® RS100 orethylcellulose. In one embodiment however the vehicle is not Eudragit®RS alone.

It is believed that the present method can be used to formmicroparticles of a wide range of drugs.

The solvent (internal phase) is preferably pure ethanol, but a varietyof mixtures of organic solvents may also be utilised, depending on thesolubility of drug and polymer. The non-solvent (external phase) ispreferably liquid paraffin.

We believe we have developed a novel emulsification/solvent evaporationmethod, allowing the fabrication inter alia of drug loaded pH-responsivepolymeric particles of Eudragit L100-55, L100, S100 and P4135 andmixtures of the polymers. We have demonstrated the successfulmicroencapsulation of the water insoluble polymers Eudragit RS100 andethylcellulose.

We have demonstrated the usefulness of Arlacel 83 (sorbitansesquioleate) for the production of Eudragit L100 and S100 particles inparticular, and believe our method is superior to other literaturemethods, in terms of its simplicity and possibility for future scale-up,as well as quality of the final product.

The particles are an ideal size for oral delivery (in the size range30-50 μm) and the excellent morphology should impart good flowproperties allowing efficient and reproducible capsule filling. Theparticles may also be suitable for administration using a bufferedsuspension.

We have demonstrated a pH-responsive release profile for EudragitL100-55, L100 and S100 in-vitro. Drug release is minimal from allpH-responsive microparticles at gastric pH, but rapid above thethreshold of the polymers. A pH-change method has been used tocharacterise drug release from L100 and S100 microparticles. The drugloading can be manipulated so that less than 10% release occurs after 2hours in acid, while the time for 100% drug release is less than 5minutes once pH is raised to intestinal/colonic levels for L100 and S100microparticles respectively.

In a second aspect of the present invention, there is provided acomposition of microparticles obtainable by means of a method as definedabove.

In a third aspect of the present invention, there is provided a methodof medical treatment comprising administering to a patient an effectiveamount of said microparticles.

In a fourth aspect of the present invention, there is provided a methodof producing microparticles comprising a bioactive and a vehicle, whichmethod comprises providing a solvent having a bioactive dispersed ordissolved therein and a vehicle dissolved therein, carrying out anemulsification to produce an emulsion of microparticles comprising thebioactive and the vehicle in a solvent phase, and evaporating thesolvent to leave said microparticles, wherein sorbitan dioleate isemployed to stabilise said emulsion.

We have entrapped a number of model drugs with different physicochemicalproperties with good efficiency, but believe the method to be capable ofmicroencapsulating a wide range of pharmaceutical agents. Encapsulationof protein and peptide drugs may also be possible, and these labiledrugs are less likely to be deactivated by this formulation method thanmore traditional tableting or pelletisation methods.

The chemicals used in the process are all widely available, relativelyinexpensive and safe. We have shown microencapsulation to be possibleusing a mixture of organic solvents and, preferably, ethanol alone thusavoiding the use of more toxic solvents. The equipment used in theprocess is also widely available.

At present no method exists for the large-scale production of EudragitL100 and S100 particles. Spray-drying has proved unsuccessful due to thethermoplastic nature of the polymers, and its tendency to form stringyaggregates. This leaves the method we have developed as the mostfeasible alternative.

REFERENCES

-   Goto, S., Kawata, M., Nakamura, M., Maekawa, K., Aoyama, T. (1986)    Eudragit E, L and S (acrylic resins) microcapsules as pH sensitive    release preparations of ketoprofen. J. Microencapsulation 3(4),    305-316.-   Morishita, I., Morishita, M., Machida, Y., Nagai, T. (1991)    Controlled release microspheres based on Eudragit L100 for the oral    administration of erythromycin. Drug Design and Delivery 7, 309-319.-   Squillante, E., Morshed, G., Bagchi, S., Mehta, K. A. (2003)    Microencapsulation of β-galactosidase with Eudragit L100. J.    Microencapsulation 20(2), 153-167.

A number of preferred embodiments of the present invention will now bedisclosed, with reference to the following drawings:

FIGS. 1 to 8 and 18 show various scanning electron micrographs (SEMs) ofexamples and comparative examples of microparticles of drug/polymermixtures;

FIGS. 9 to 17 show drug release profiles from various microparticlesmade in accordance with the invention.

Preliminary experiments using Span 85 as a surfactant were carried outto optimise the choice of solvent mixture. 30 mL mixtures of acetone andeither ethanol or methanol in different ratios were tried, and it wasfound that acetone/methanol mixtures worked better than acetone/ethanol,probably due to a faster evaporation of methanol resulting from a lowerboiling point and reduced affinity for the polymer, Eudragit S100. Whenmethanol alone was used, large, hollow, and sometimes, cracked particleswere produced. Acetone alone did not produce any microparticles.Increasing the proportion of acetone reduced the size but seemed toincrease the degree of aggregation. 20 mL acetone/10 mL methanol was theoptimal solvent mixture as judged by SEM analysis, and it was decided touse this in future experiments, and change the surfactant.

We decided to investigate surfactants with an HLB in the range 1 to 10.Surfactants in, and close to, this range were therefore sourced, and asimple system using liquid paraffin as non-solvent was tried, withoverhead propeller stirring from a Heidolph RZR1 stirrer calibrated to1000 rpm.

A mixture of 30 mL acetone/methanol (2:1) was used to dissolve 3 gramsEudragit S100 polymer. No drug was added in these experiments as theywere intended to investigate microparticle formation only. Stirring andsolvent evaporation were allowed to proceed overnight, and the productwas collected by vacuum filtration through a sintered glass filter thenext day, washed with three 50 ml portions of hexane to remove traces ofliquid paraffin, and dried in a vacuum oven for 24 hours.

All experiments were carried out in triplicate, and the polymer used inthe optimisation process was always Eudragit S100.

The following surfactants were initially employed at 1% concentration,and 2 and 3% if necessary; Span 85 (HLB 1.8), Span 80 (HLB 4.3), Span 20(HLB 8.6), Brij 92 (HLB 4.9), Brij 52 (HLB 5.3) and sorbitansesquioleate (Arlacel 83) (HLB 3.7).

Particles were firstly examined by optical microscopy (Nikon MicrophotFXA) at ×4 and ×10 objective magnification, and images were capturedusing a JVC video camera. An indication of the overall morphology anddegree of aggregation was possible, but to observe the surfacecharacteristics of the microparticles in detail, SEMs of promisingparticles were taken. The microspheres were fixed on SEM adhesion pads,and coated with gold using a gold sputter module in a high-vacuumevaporator (Emitech K550). Samples were examined with the scanningelectron microscope (Phillips XL30 TMP) at 10 kV.

Results COMPARATIVE EXAMPLE 1 Use of Span Surfactants

When Span 85 was used as surfactant, Eudragit S100 appeared to produceaggregated particles when viewed under the optical microscope. SEManalysis confirmed the presence of semi-formed, aggregated particles,possibly originating from particle coalescence during solventevaporation (see FIG. 1). It can be concluded that Span 85 does notstabilise the emulsion sufficiently to allow formation of discreetmicroparticles.

Span 80 produced large non-particulate lumps of polymer, larger than 1mm in diameter and no further analysis of the product was required.

Span 20 produced thin spindle-like polymeric fibres (data not shown),again no further analysis was necessary.

Span 65 is a cream/yellow solid at room temperature, and was immisciblewith liquid paraffin after heating. Furthermore it did not dissolve inthe acetone/methanol mixture, and therefore was unable to stabilise theemulsion and produce microparticles.

COMPARATIVE EXAMPLE 2 User of Brij Surfactants

Two Brijs were then tried with appropriate HLB values; Brij 52 with HLB5.3 and Brij 92 with HLB 4.9. Brij 52 is a waxy solid at roomtemperature, Brij 92 is a liquid. Brij 52 and 92 were tried at 1, 2 and3% concentrations.

At 1% Brij 92 concentration, again we see aggregates of semi-formedspherical particles. Increasing the concentration to 2 and 3% does nothave a positive influence on microparticle morphology, and themorphology of the 3% sample seems to be the worst of the 3 samples (seeFIGS. 2A, 2B and 2C).

At room temperature, Brij 52 was a solid and not miscible with liquidparaffin, but upon heating 1% Brij 52 could be dissolved into liquidparaffin and did not precipitate out on cooling. It was soluble in themixture of acetone/methanol. Both these formulation methods producedaggregated particles as seen in FIGS. 2D and 2E.

Heating to incorporate Brij 52 into the liquid paraffin phase appears tobe preferable to dissolving the surfactant in the internal phase, as canbe seen from FIGS. 2D and 2E. Particles produced using the formerformulation strategy are more spherical in appearance, less polydisperseand of smaller size. However, they are still aggregated.

EXAMPLE 3 Use of Sorbitan Sesquioleate (Arlacel 83) as a Surfactant

The surfactant Arlacel 83 was incorporated into the system inconcentrations of 1, 2 and 3% (see FIGS. 3A, 3B and 3C). Arlacel 83 is asorbitan fatty acid ester, similar to the Spans, with an HLB value of3.7, and is an equimolar mixture of sorbitan monooleate and sorbitandioleate. In all concentrations it had a dramatic effect on theappearance of the microspheres, producing spherical, non-aggregated,non-porous particles in the required size range. The samples alsoappeared to be monodisperse.

EXAMPLE 4

Aim: To investigate the formation of microparticles using a variety ofsurfactants with combined HLBs of about 3.7.

Method: A mixture of 14.4% Tween 80 and 85.6% Span 85, and 56.5% Span 80and 43.5% Span 85 were added to liquid paraffin in a 1% concentration.Both mixtures have an HLB value of 3.7, the same as Arlacel 83. Theemulsification/solvent evaporation method was carried out as before.

Results: The SEM of the particles is shown below in FIGS. 4A and 4B.

Conclusions: A mixture of two surfactants with a combined HLB of 3.7seems capable of stabilising the emulsion to facilitate the productionof microparticles of acceptable morphology.

EXAMPLE 5

The proposed method of microencapsulation can also be used for EudragitL100 and mixtures of L100 and S100

Aim: To produce microparticles of Eudragit L100 and mixed EudragitL/S100 using Arlacel 83.

Method: 3 grams of Eudragit L100 and 3 grams of a 1:1 mixture ofEudragit L100 and S100 were dissolved in 30 mL acetone/methanol 1:1 aspreviously, and emulsified into 200 ml liquid paraffin containing 1%Arlacel 83.

Results: On both occasions, particles of excellent morphology wereformed, comparable to Eudragit S100 particles (see FIG. 5). It shouldnow be possible to formulate microparticles that will release drug at apH of between 6.0 and 6.8.

EXAMPLE 6

The next step was to try solvent mixtures that had worked best when span85 was used as a surfactant.

Aim: To study the effect of internal phase solvent on the morphology ofEudragit S100 microparticles produced by the emulsification/solventevaporation method.

Method:

Experiments were conducted as before, but dissolving 3 grams of EudragitS100 in different mixtures of acetone and either methanol or ethanol. 1%Arlacel 83 in 200 mL liquid paraffin was used as external phase. Thesolvent compositions used are shown in the table below, and were decidedfrom preliminary experiments using span 85 in liquid paraffin, in whichwe discovered solvent compositions in the range 1:1 to 5:1alcohol/acetone were optimal, and could influence microparticlemorphology, with acetone/methanol mixtures being more effective thanacetone/ethanol.

Sample Internal phase solvent 1 15 mL acetone/15 mL methanol 2 15 mLacetone/15 mL ethanol 3 20 mL acetone/10 mL methanol 4 20 mL acetone/10mL ethanol 5 25 mL acetone/5 mL methanol 6 25 mL acetone/5 mL ethanol

Results: The SEMs from the above experiments are shown in FIGS. 6A to6E.

Conclusions: From the results of the above experiment, variouscombinations of acetone and either methanol or ethanol have allowed theformation of Eudragit S100 microparticles with excellent morphology. Thechoice of solvent has little effect on the morphology of themicroparticles This provides evidence that the choice of surfactant ismore crucial than the choice of polymer solvent. All particles areunaggregated, non-porous and in the desired size range.

EXAMPLE 7 Use of Ethanol as a Sole Solvent for the Production ofMicroparticles of L100, S100 and L100-55

It would be desirable to produce particles using only ethanol asdisperse phase solvent, to simplify the method of production and toreduce toxicity concerns due to any residual solvent in themicroparticles, ethanol being less toxic than acetone and methanol.Therefore, 30 mL portions of ethanol were used to dissolve 3 grams ofL100-55, L100 and S100. The emulsification/solvent evaporation was usedas before, with 200 ml liquid paraffin containing 1% w/w Arlacel 83 assurfactant. SEMs of the microparticles are shown in FIGS. 7A to 7C.

Conclusions

The SEMs show that for L100-55, L100 and S100 particles of excellentmorphology can be used using a single, relatively non-toxic solvent.

EXAMPLE 8 Use of Eudragit RS100 and Mixtures of RS100 with L100 and S100

Microencapsulation of the water insoluble polymer Eudragit RS100 wastried alone, and in combination with L100 and S100. RS100 alone can beused for sustained release applications, and in combinations with thepH-sensitive Eudragits may modify the release from these polymers. 3grams RS100 was soluble in 30 mL acetone and 30 ml acetone/ethanol(1:1), but not 30 mL ethanol. 1:1 mixtures of RS100/L100 and RS100/S100were soluble in all three solvent mixtures. The SEMs of the productsobtained are shown in FIGS. 8A-8H.

Conclusions

The SEMs show that the method produces good particles when RS100 is usedalone, or in combination with L100 or S100. It is expected that RS100will retard the release of drug from a pH-responsive microparticulatesystem. The RS/S combination may allow for a sustained release of drugin the colonic region, as opposed to dose-dumping which may occur from apurely pH-responsive system. It is foreseeable that such a system wouldhave benefits in the topical therapy of inflammatory bowel diseases,preventing a total premature release of drug and systemic absorption,but would be unlikely to be voided before significant drug release hadoccurred due to the prolonged colonic retention of small particulatesystems.

Similarly, the mixture of RS/L may permit a controlled releasethroughout the length of the small intestine. Particles formed fromRS100 may have sustained release applications, and also show theversatility of our method of microencapsulation, particularly for theEudragit range of polymers.

Proof of Concept: In-Vitro Drug Release Profiles EXAMPLES 9 TO 15

FIGS. 9 to 14 show the following in-vitro drug release profiles formicroparticles in different pH media, using USPII paddle apparatus. Allthe microparticles were formed using Arlacel 83 as a surfactant.

FIG. 9 shows prednisolone release from Eudragit L100 (10:1) particles atpH 1.2-6.8. This is an averaged profile of a series of six differentexamples.

FIG. 10 shows a comparison of prednisolone release from Eudragit S100microparticles and an equivalent S100 coated tablet system at pH1.2-7.4.

FIG. 11 is a comparison of prednisolone release from Eudragit S100microparticles with different drug loadings.

FIG. 12 shows prednisolone release from Eudragit RS/S microparticles(1:1) at pH 1.2-7.4 to demonstrate that water-insoluble Eudragit RSsustains release from S100 particles at colonic pH.

FIG. 13 is a release profile for 6 batches of Eudragit S100/prednisolone(5:1) microparticles at pH 1.2-7.4 which demonstrates batch to batchreproducibility.

FIG. 14 is a profile showing prednisolone release from Eudragit RS/S(1:1) microparticles at gastric pH for 2 hours, proximal intestinal pHfor 1 hour, and colonic pH for 2 hours. Little prednisolone release isseen for the first 3 hours, but when the pH is changed to 7.4 themajority of the drug is released over a period of about an hour. This isan averaged profile of four different samples.

FIG. 15 is a release profile comparing RS/S100 (50:50) at pH 1.2-7.4,with ethylcellulose/S100 (50:50). This is essentially comparing theability of two water insoluble polymers, each in combination with S100,to achieve sustained release profiles. Ethylcellulose seems to performbetter, but mixing different proportions of either water insolublepolymer will give different tailored release profiles.

EXAMPLE 16

Surfactants were now mixed to identify the HLB range within which 2surfactants would work together to stabilise the emulsion and produceacceptable microparticles:

(a) 50% Span 80/50% Span 85 (HLB 3) (b) 53% Span 85/47% Span 20 (HLB 5)(c) 60% Span 80/40% Span 20 (HLB 6) (d) 35% Span 80/65% Span 20 (HLB 7).

(HLB values are given to 1 significant figure above).

All were used as previously at 1% w/w concentration. Microparticles wereexamined by microscopy and adjudged to have acceptable morphology. Drugloaded particles were prepared (5:1 Eudragit S/prednisolone) andin-vitro drug release evaluated (pH 1.2 for 2 hours, raised to pH 7.4).

All particles behaved as dictated by the dissolution threshold pH of thepolymer, i.e. little drug release after 2 hours incubation in acid,rapid and complete drug release after pH change (see FIG. 16).

EXAMPLE 17

FIG. 17 shows a comparison of bendroflumethazide, prednisolone andbudesonide release from Eudragit S100 microparticles at pH 1.2-7.4. Thisdemonstrates that almost all of the loaded drug is released rapidly asthe pH changes from gastric to colonic pH for all three drugs.

EXAMPLE 18

FIGS. 18A, 18B and 18C show SEMs of microparticles produced using 1%Arlacel 83 as a surfactant and Ethylcellulose N100, Hydroxypropylmethylcellulose phthalate (HPMCP50) and Polyvinyl acetate phthalate(PVAP) as the polymer in each case respectively. No drug dissolutionwork has been done with these microparticles, and indeed the PVAPmicroparticles are likely to be too large to fall within the terms ofthe invention. Nevertheless, the results are included to demonstratethat microparticles can be formed with cellulose-based andpolyvinyl-based polymers.

1. A method of producing microparticles comprising a bioactive and a vehicle, which method comprises providing a solvent having a bioactive dispersed or dissolved therein and a vehicle dissolved therein, carrying out an emulsification in a non-solvent phase to produce an emulsion comprising the bioactive and the vehicle in a solvent phase, and evaporating the solvent to leave said microparticles, wherein a mixture of at least two surfactants is employed to stabilise said emulsion and the HLB (hydrophilic-lipophilic balance) of the mixture is up to 8 in order that the median diameter of the microparticles is up to 100 μm.
 2. A method as claimed in claim 1, wherein said HLB is from 2 to
 7. 3. A method as claimed in claim 1 or 2, wherein said HLB is from 3 to
 5. 4. A method as claimed in any preceding claim, wherein said HLB is from 3 to
 4. 5. A method as claimed in any preceding claim, wherein said mixture comprises sorbitan monoleate and sorbitan dioleate.
 6. A method as claimed in any preceding claim, wherein said mixture is an equimolar mixture of two surfactants.
 7. A method as claimed in any preceding claim, wherein the vehicle is a polymer which enables pH-dependent and/or pH-independent release of the bioactive in the gastrointestinal tract.
 8. A method as claimed in any of claims 1 to 6, wherein the vehicle is a polymer which enables pH-dependent release of the bioactive in the gastrointestinal tract.
 9. A method as claimed in any preceding claim, wherein the vehicle is an acrylic-based polymer, a cellulose-based polymer or a polyvinyl-based polymer.
 10. A method as claimed in claim 9, wherein the vehicle is a methacrylate polymer.
 11. A method as claimed in any preceding claim, wherein the vehicle comprises Eudragit® L100, Eudragit® L100-55, Eudragit® S100, Eudragit® P4135, Eudragit® RS100 or ethylcellulose.
 12. A method as claimed in any of claims 1 to 8, wherein the vehicle is not Eudragit® RS alone.
 13. A method as claimed in any preceding claim, wherein the bioactive is prednisolone, bendrofluazide or budesonide.
 14. A method as claimed in any preceding claim, wherein the solvent is ethanol or a mixture of acetone and ethanol or methanol.
 15. A method as claimed in any preceding claim, wherein the surfactants in said mixture are both added to the solvent phase, both added to the non-solvent phase, or wherein one is added to each phase.
 16. A method as claimed in any preceding claim, wherein the non-solvent phase is liquid paraffin.
 17. A method as claimed in any preceding claim, wherein the emulsification is carried out at a temperature from 10 to 30° C.
 18. A composition of microparticles obtainable by means of a method as claimed in any preceding claim.
 19. A method of medical treatment comprising administering to a patient an effective amount of microparticles as claimed in claim
 18. 